The present invention relates to a chain saw having a longitudinally displaceable guide bar for varying the tension on the saw chain.
For proper operation of a chain saw the saw chain has to be tight with a proper tensioning force. The guide bar is during operation clamped to the motor unit of the chain saw machine by a clamping plate which is normally held by two nuts threaded onto two bolts fixed to the motor unit and penetrating through a slot in the guide bar. When the chain tension is to be adjusted, e.g., when a chain is worn or when mounting a new chain, the nuts are left slightly loose, and the guide bar is pulled outwards by hand, or by some mechanism actuated by screwdrivers or wheels. Such mechanisms, which are usually located in the motor unit as described in the following patents: U.S. Pat. No. 5,528,835, No. 5,353,506, No. 5,491,899, German Document No. 42 22075, U.S. Pat. No. 5,497,557 and No. G-93 11081, push the guide bar outwards, e.g., by a peg penetrating a hole in the guide bar which is offset to one side of the bolt slot. To move the peg many different mechanisms have been used, employing helical or conical gears, cams or angular levers.
One main problem has been, however, to reach the actuating means. Since the strength of the guide bar in this critical region does not allow any extra cutouts to be formed therein, most designs involve an actuating means accessible behind the guide bar, often only by turning the chain saw upside down, which makes it difficult and time-consuming to adjust the chain tension. In some cases it has been attempted to control the peg movement from the same side as the nuts, such as German Document 44 36300 where the peg itself is rotated and connected by conical gears to a nut on a threaded shaft, which requires a special clamping plate with an oblong hole and which presents difficulties to arrange suitable bearings for the rotatable peg. In German Document G-93 11081, one of the nuts is connected by hollow spline shifts, helical gears and slip clutches to the screw mechanism for moving the peg, which requires an extra wide bolt slot to accommodate the hollow spline shaft.
All those mechanisms are rather complicated, employing a multitude of components and making the shell of the motor unit larger and difficult to produce. They also share the disadvantage that before fitting a new chain or guide bar, the peg has to be reset rearward.
Another type of tensioning mechanism is described in U.S. Pat. Nos. 4,939,842 and 5,070,618 where a gear wheel with a shaft journalled in the clamping plate acts on a rack part of the bolt slot, or a gear wheel with a shaft without any proper journal but guided by a hole in the guide bar acts on a rack surface in a recess in the motor casing. In both cases the shaft is rotated with a screwdriver. Disadvantages involve an insufficient journal support of the shaft, and the need of a relatively large gear diameter which requires a strong hand torque. The latter patent requires a difficult machining of the recess in the motor casing.